It happens. The audience just is not responding to you. It could be the subject matter, the speaker, or the audience is tired and uninspired. Who knows?
You’ll be tempted to crack jokes and lighten up the delivery. Don’t. Continue to make eye contact. Smile, but stay serious and focused. Don’t rush. Finish according to plan.
It’s impossible to Wow the audience every time. There are too many moving parts, many of which are out of your control.